Method and apparatus for sensing environment using a wireless passive sensor

ABSTRACT

A method for execution by a RFID tag includes receiving, from an RFID reader, an RF signal, where the RF signal has a carrier frequency of a plurality of carrier frequencies that span a broad frequency band. The method further includes adjusting input impedance of the RFID tag over a range of input impedances, where the input impedance of the RFID tag is based on one or more of impedance of the RFID tag&#39;s antenna and tank circuit. The method further includes monitoring power level of the received RF signal over the range of input impedances. The method further includes selecting the input impedance of the range of input impedances providing a substantially maximum power level of the received RF signal, where the substantially maximum power level corresponds to the carrier frequency being substantially equal to a resonant frequency of at least one of the antenna and the tank circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present U.S. Utility patent application claims priority pursuant to35 U.S.C. §120 as a Continuation of U.S. Utility application Ser. No.14/256,877, entitled “METHOD AND APPARATUS FOR SENSING ENVIRONMENT USINGA WIRELESS PASSIVE SENSOR”, filed 18 Apr. 2014, which is incorporatedherein by reference in its entirety and made part of the present U.S.Utility patent application for all purposes, and said Ser. No.14/256,877 is a Continuation-In-Part of application Ser. No. 13/209,420,filed 14 Aug. 2011 (“Parent Application One”), now U.S. Pat. No.8,749,319, issued on 10 Jun. 2014, which claims priority to U.S.Provisional Application Ser. No. 61/428,170, filed 29 Dec. 2010 (“ParentProvisional One”) and U.S. Provisional Application Ser. No. 61/485,732,filed 13 May 2011 (“Parent Provisional Two”). Parent Application One(Ser. No. 13/209,420) is, in turn, a Continuation-In-Part of applicationSer. No. 12/462,331, filed 1 Aug. 2009, which is now U.S. Pat. No.8,081,043, issued 20 Dec. 2011 (“Parent Patent One”), which is aDivisional of U.S. Utility application Ser. No. 11/601,085, filed 18Nov. 2006, now U.S. Pat. No. 7,586,385, issued on 8 Sep. 2009.

U.S. Utility application Ser. No. 14/256,877 is also aContinuation-In-Part of application Ser. No. 13/467,925, filed 9 May2012 (“Parent Application Two”), which is a Continuation-in-Part of U.S.Utility application Ser. No. 13/209,425, filed 14 Aug. 2011, now U.S.Pat. No. 9,048,819, issued on 2 Jun. 2015, which claims prioritypursuant to 35 U.S.C. §119(e) to U.S. Provisional Application No.61/428,170, filed 29 Dec. 2010 and U.S. Provisional Application No.61/485,732, filed 13 May 2011, and U.S. Utility application Ser. No.13/209,425 also claims priority pursuant to 35 U.S.C. §120 as aContinuation-in-Part of U.S. Utility application Ser. No. 12/462,331,filed 1 Aug. 2009, now U.S. Pat. No. 8,081,043, issued on 20 Dec. 2011,which is a Divisional of U.S. Utility application Ser. No. 11/601,085,filed 18 Nov. 2006, now U.S. Pat. No. 7,586,385, issued on 8 September2009.

U.S. Utility application Ser. No. 14/256,877 is also aContinuation-In-Part of application Ser. No. 13/209,425, filedsimultaneously with the Parent Application One on 14 Aug. 2011 (“RelatedCo-application”), now U.S. Pat. No. 9,048,819, issued on 2 Jun. 2015,which claims priority pursuant to 35 U.S.C. §119(e) to U.S. ProvisionalApplication No. 61/428,170, filed 29 Dec. 2010 and U.S. ProvisionalApplication No. 61/485,732, filed 13 May 2011, and U.S. Utilityapplication Ser. No. 13/209,425 also claims priority pursuant to 35U.S.C. §120 as a Continuation-in-Part of U.S. Utility application Ser.No. 12/462,331, filed 1 Aug. 2009, now U.S. Pat. No. 8,081,043, issuedon 20 Dec. 2011, which is a Divisional of U.S. Utility application Ser.No. 11/601,085, filed 18 Nov. 2006, now U.S. Pat. No. 7,586,385, issuedon 8 Sep. 2009.

U.S. Utility application Ser. No. 14/256,877 also claims priority to:

1. U.S. Provisional Application Ser. No. 61/814,241, filed 20 Apr. 2013,(“Parent Provisional Three”);2. U.S. Provisional Application Ser. No. 61/833,150, filed 10 Jun. 2013,(“Parent Provisional Four”);3. U.S. Provisional Application Ser. No. 61/833,167, filed 10 Jun. 2013,(“Parent Provisional Five”);4. U.S. Provisional Application Ser. No. 61/833,265, filed 10 Jun. 2013,(“Parent Provisional Six”);5. U.S. Provisional Application Ser. No. 61/871,167, filed 28 Aug. 2013,(“Parent Provisional Seven”);6. U.S. Provisional Application Ser. No. 61/875,599, filed 9 Sep. 2013,(“Parent Provisional Eight”);7. U.S. Provisional Application Ser. No. 61/896,102, filed 27 Oct. 2013,(“Parent Provisional Nine”);8. U.S. Provisional Application Ser. No. 61/929,017, filed 18 Jan. 2014,(“Parent Provisional Ten”);9. U.S. Provisional Application Ser. No. 61/934,935, filed 3 Feb. 2014,(“Parent Provisional Eleven”);collectively, “Parent Provisional References”, and hereby claims benefitof the filing dates thereof pursuant to 37 CFR §1.78(a)(4).

The subject matter of the Parent Applications One, Two and Three, ParentPatent One, the Related Co-application, and the Parent ProvisionalReferences (collectively, “Related References”), each in its entirety,is expressly incorporated herein by reference.

BACKGROUND OF THE INVENTION Technical Field of the Invention

The present invention relates generally to sensing a detectableenvironmental condition, and, in particular, to sensing a detectableenvironmental condition in a passive RFID system.

Description of Related Art

In general, in the descriptions that follow, we will italicize the firstoccurrence of each special term of art that should be familiar to thoseskilled in the art of radio frequency (“RF”) communication systems. Inaddition, when we first introduce a term that we believe to be new orthat we will use in a context that we believe to be new, we will boldthe term and provide the definition that we intend to apply to thatterm. In addition, throughout this description, we will sometimes usethe terms assert and negate when referring to the rendering of a signal,signal flag, status bit, or similar apparatus into its logically true orlogically false state, respectively, and the term toggle to indicate thelogical inversion of a signal from one logical state to the other.Alternatively, we may refer to the mutually exclusive boolean states aslogic_0 and logic_1. Of course, as is well known, consistent systemoperation can be obtained by reversing the logic sense of all suchsignals, such that signals described herein as logically true becomelogically false and vice versa. Furthermore, it is of no relevance insuch systems which specific voltage levels are selected to representeach of the logic states.

In accordance with our prior invention previously disclosed in theRelated References, the amplitude modulated (“AM”) signal broadcast bythe reader in an RFID system will be electromagnetically coupled to aconventional antenna, and a portion of the current induced in a tankcircuit is extracted by a regulator to provide operating power for allother circuits. Once sufficient stable power is available, the regulatorwill produce, e.g., a power-on-reset signal to initiate systemoperation. Thereafter, the method disclosed in the Related References,and the associated apparatus, dynamically varies the capacitance of avariable capacitor component of the tank circuit so as to dynamicallyshift the f_(R) of the tank circuit to better match the f_(C) of thereceived RF signal, thus obtaining maximum power transfer in the system.

In general, the invention disclosed in the Related References focusedprimarily on quantizing the voltage developed by the tank circuit as theprimary means of matching the f_(R) of the tank circuit to thetransmission frequency, f_(C), of the received signal. However, thisvoltage quantization is, at best, indirectly related to received signalfield strength. In the First Related Application, we disclosed aneffective and efficient method and apparatus for quantizing the receivedfield strength as a function of induced current. In particular, wedisclosed a method and apparatus adapted to develop this fieldquantization in a form and manner that is suitable for selectivelyvarying the input impedance of the receiver circuit to maximize receivedpower, especially during normal system operation. Additionally, in lightof the power sensitive nature of RFID systems, our disclosed method andapparatus varied the input impedance with a minimum power loss.

In Parent Application One, we have disclosed generally the use of ourmethod and apparatus to sense changes to an environment to which theRFID tag is exposed. In this application, we will further develop thiscapability and disclose embodiments specifically adapted to operate in avariety of environments.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates, in block diagram form, an RF receiver circuit havinga field strength detector constructed in accordance with an embodimentof our invention;

FIG. 2 illustrates, in block diagram form, a field strength detectorcircuit constructed in accordance with an embodiment of our invention;

FIG. 3 illustrates, in block schematic form, a more detailed embodimentof the field strength detector circuit shown in FIG. 2;

FIG. 4 illustrates, in flow diagram form, the sequencing of operationsin the field strength detector circuit shown in FIG. 3;

FIG. 5 illustrates, in graph form, the response of the field strengthdetector circuit shown in FIG. 3 to various conditions;

FIG. 6 illustrates, in block schematic form, an RF receiver circuitconstructed in accordance with another embodiment of our invention;

FIG. 7 illustrates, in flow diagram form, the sequencing of theoperations in the RF receiver circuit shown in FIG. 6;

FIG. 8 illustrates, in block schematic form, an alternativerepresentation of the impedance represented by the antenna and the tankcircuit of the exemplary RFID receiver circuit;

FIG. 9 illustrates, in block schematic form, an alternative exemplaryembodiment of the field strength detector circuit shown in FIG. 3;

FIG. 10 illustrates, in block schematic form, an alternative exemplaryembodiment of the field strength detector circuit shown in FIG. 3;

FIG. 11 illustrates, in block schematic form, an exemplary RFIDsub-system containing tag and reader;

FIG. 12 illustrates, in flow diagram form, the sequencing of theoperations in developing a reference table associating tank tuningparameters with system frequency;

FIG. 13, comprising FIGS. 13a and 13b , illustrates an RF systemconstructed in accordance with one embodiment of our invention to senseenvironmental conditions in a selected region surrounding the system;

FIG. 14 illustrates, in perspective, exploded view, one possibleconfiguration of an antenna and tail arrangement adapted for use in thesystem of FIG. 13;

FIG. 15, comprising FIG. 15a through FIG. 15h , illustrates an antennaconstructed in accordance with one embodiment of the present invention,wherein: FIG. 15a illustrates in top plan view a fully assembledantenna; FIG. 15b and FIG. 15c illustrate, in cross-section, the severallayers comprising a head and a tail portion, respectively, of theantenna; FIG. 15d through FIG. 15g illustrate, in plan view, the severalseparate layers of the antenna as shown in FIG. 15b and FIG. 15c ; andFIG. 15h illustrates, in partial plan view, a close-up depiction of acentral, slot portion of the antenna of FIG. 15a (as noted in FIG. 15e )showing in greater detail the construction of antenna elements to whichan RFID tag die may be attached;

FIG. 16 illustrates, in flow diagram form, the sequencing of theoperations in detecting the presence of a contaminant using, e.g., theantenna of FIG. 15 in the system shown in FIG. 11; and

FIG. 17, comprising FIG. 17a and FIG. 17b , illustrates a folded, patchantenna constructed in accordance with one other embodiment of thepresent invention, wherein: FIG. 17a illustrates, in plan view, the toplayer of the antenna after placement of the RFID tag die but beforefolding along fold lines 1 and 2; and FIG. 17b illustrates, also in planview, the bottom layer of the antenna as shown in FIG. 17 a.

In the drawings, similar elements will be similarly numbered wheneverpossible. However, this practice is simply for convenience of referenceand to avoid unnecessary proliferation of numbers, and is not intendedto imply or suggest that our invention requires identity in eitherfunction or structure in the several embodiments.

DETAILED DESCRIPTION OF THE INVENTION

Shown in FIG. 1 is an RF receiver circuit 10 suitable for use in an RFIDapplication. As we have described in our Related References, an RFsignal electromagnetically coupled to an antenna 12 is received via atank circuit 14, the response frequency, f_(R), of which is dynamicallyvaried by a tuner 16 to better match the transmission frequency, f_(C),of the received RF signal, thus obtaining a maximum power transfer. Inparticular, as further noted in the Related Applications, the RMSvoltage induced across the tank circuit 14 by the received RF signal isquantized by tuner 16 and the developed quantization employed to controlthe impedance of the tank circuit 14. As also described in the RelatedReferences, the unregulated, AC current induced in the tank circuit 14by the received RF signal is conditioned by a regulator 18 to provideregulated DC operating power to the receiver circuit 10. In accordancewith our present invention, we now provide a field strength detector 20,also known as a power detector, adapted to develop a field-strengthvalue as a function of the field strength of the received RF signal. Aswe have indicated in FIG. 1, our field strength detector 20 is adaptedto cooperate with the regulator 18 in the development of thefield-strength value. As we shall disclose below, if desired, our fieldstrength detector 20 can be adapted to cooperate with the tuner 16 incontrolling the operating characteristics of the tank circuit 14.

Shown by way of example in FIG. 2 is one possible embodiment of ourfield strength or power detector 20. In this embodiment, we have chosento employ a shunt-type regulator 18 so that, during normal operation, wecan use the shunted ‘excess’ current as a reference against which wedevelop the field-strength value. In this regard, we use a reference 22first to develop a shunt current reference value proportional to theshunted current, and then to develop a mirrored current reference valueas a function of both the shunted current and a field strength referencecurrent provided by a digitally-controlled current source 24.Preferably, once the tuner 16 has completed its initial operatingsequence, whereby the f_(R) of the tank circuit 14 has beensubstantially matched to the f_(C) of the received signal, we thenenable a digital control 26 to initiate operation of the current source24 at a predetermined, digitally-established minimum field strengthreference current. After a predetermined period of time, control 26captures the mirrored current reference value provided by the currentreference 22, compares the captured signal against a predeterminedthreshold value, and, if the comparison indicates that the fieldstrength reference current is insufficient, increases, in accordancewith a predetermined sequence of digital-controlled increments, thefield strength reference current; upon the comparison indicating thatthe field strength reference current is sufficient, control 26 will, atleast temporarily, cease operation.

In accordance with our invention, the digital field-strength valuedeveloped by control 26 to control the field strength current source 24is a function of the current induced in the tank circuit 14 by thereceived RF signal. Once developed, this digital field-strength valuecan be employed in various ways. For example, it can be selectivelytransmitted by the RFID device (using conventional means) back to thereader (not shown) for reference purposes. Such a transaction can beeither on-demand or periodic depending on system requirements. Imaginefor a moment an application wherein a plurality of RFID tag devices aredistributed, perhaps randomly, throughout a restricted, 3-dimensionalspace, e.g., a loaded pallet. Imagine also that the reader is programmedto query, at an initial field strength, all tags “in bulk” and tocommand all tags that have developed a field-strength value greater thana respective field-strength value to remain ‘silent.’ By performing asequence of such operations, each at an increasing field strength, thereader will, ultimately, be able to isolate and distinguish those tagsmost deeply embedded within the space; once these ‘core’ tags have beenread, a reverse sequence can be performed to isolate and distinguish alltags within respective, concentric ‘shells’ comprising the space ofinterest. Although, in all likelihood, these shells will not be regularin either shape or relative volume, the analogy should still be apt.

In FIG. 3, we have illustrated one possible embodiment of our fieldstrength detector 20 a. In general, we have chosen to use a shuntcircuit 18 a to develop a substantially constant operating voltage levelacross supply node 28 and ground node 30. Shunt regulators of this typeare well known in the art, and typically use zener diodes, avalanchebreakdown diodes, diode-connected MOS devices, and the like.

As can be seen, we have chosen to implement current reference 22 in theform of a current mirror circuit 22 a, connected in series with shuntcircuit 18 a between nodes 28 and 30. As is typical, current mirrorcircuit 22 a comprises a diode-connected reference transistor 32 and amirror transistor 34. If desired, a more sophisticated circuit such as aWidlar current source may be used rather than this basic two-transistorconfiguration. For convenience of reference, we have designated thecurrent shunted by shunt circuit 18 a via reference transistor 32 as ix;similarly, we have designated the current flowing through mirrortransistor 34 as ix/N, wherein, as is known, N is the ratio of thewidths of reference transistor 32 and mirror transistor 34.

We have chosen to implement the field strength current source 24 as aset of n individual current sources 24 a, each connected in parallelbetween the supply node 28 and the mirror transistor 34. In general,field strength current source 24 a is adapted to source current at alevel corresponding to an n-bit digital control value developed by acounter 38. In the illustrated embodiment wherein n=5, field strengthcurrent source 24 a is potentially capable of sourcing thirty-twodistinct reference current levels. We propose that the initial, minimumreference current level be selected so as to be less than the currentcarrying capacity of the mirror transistor 34 when the shunt circuit 18a first begins to shunt excess induced current through referencetransistor 32; that the maximum reference current level be selected soas to be greater than the current carrying capacity of the mirrortransistor 34 when the shunt circuit 18 a is shunting a maximumanticipated amount of excess induced current; and that the intermediatereference current levels be distributed relatively evenly between theminimum and maximum levels. Of course, alternate schemes may bepracticable, and, perhaps, desirable depending on system requirements.

Within control 26 a, a conventional analog-to-digital converter (“ADC”)40, having its input connected to a sensing node 36, provides a digitaloutput indicative of the field strength reference voltage, v_(R),developed on sensing node 36. In one embodiment, ADC 40 may comprise acomparator circuit adapted to switch from a logic_0 state to a logic_1when sufficient current is sourced by field strength current source 24 ato raise the voltage on sensing node 36 above a predetermined referencevoltage threshold, v_(th). Alternatively, ADC 40 may be implemented as amulti-bit ADC capable of providing higher precision regarding thespecific voltage developed on sensing node 36, depending on therequirements of the system. Sufficient current may be characterized asthat current sourced by the field strength current source 24 a or sunkby mirror transistor 34 such that the voltage on sensing node 36 isaltered substantially above or below a predetermined reference voltagethreshold, v_(th). In the exemplary case of a simple CMOS inverter,v_(th) is, in its simplest form, one-half of the supply voltage (VDD/2).Those skilled in the art will appreciate that v_(th) may byappropriately modified by altering the widths and lengths of the devicesof which the inverter is comprised. In the exemplary case a multi-bitADC, v_(th) may be established by design depending on the systemrequirements and furthermore, may be programmable by the system.

In the illustrated embodiment, a latch 42 captures the output state ofADC 40 in response to control signals provided by a clock/controlcircuit 44. If the captured state is logic_0, the clock/control circuit44 will change counter 38 to change the reference current being sourcedby field strength current source 24 a; otherwise clock/control circuit44 will, at least temporarily, cease operation. However,notwithstanding, the digital field-strength value developed by counter38 is available for any appropriate use, as discussed above.

By way of example, we have illustrated in FIG. 4 one possible generaloperational flow of our field strength detector 20 a. Upon activation,counter 38 is set to its initial digital field-strength value (step 48),thereby enabling field strength current source 24 a to initiatereference current sourcing at the selected level. After an appropriatesettling time, the field strength reference voltage, v_(R), developed onsensing node 36 and digitized by ADC 40 is captured in latch 42 (step50). If the captured field strength reference voltage, v_(R), is lessthan (or equal to) the predetermined reference threshold voltage,v_(th), clock/control 44 will change counter 38 (step 54). This processwill repeat, changing the reference current sourced by field strengthcurrent source 24 a until the captured field strength reference voltage,v_(R), is greater than the predetermined reference threshold voltage,v_(th), (at step 52), at which time the process will stop (step 56). Asillustrated, this sweep process can be selectively reactivated asrequired, beginning each time at either the initial field-strength valueor some other selected value within the possible range of values asdesired.

The graph illustrated in FIG. 5 depicts several plots of the voltagedeveloped on sensing node 36 as the field strength detector circuit 20 asweeps the value of counter 38 according to the flow illustrated in FIG.4. As an example, note that the curve labeled “A” in FIG. 5 begins at alogic_0 value when the value of counter 38 is at a minimum value such as“1” as an exemplary value. Subsequent loops though the sweep loopgradually increase the field strength reference voltage on sensing node36 until counter 38 reaches a value of “4” as an example. At this point,the “A” plot in FIG. 5 switches from a logic_0 value to a logic_1 value,indicating that the field strength reference voltage, v_(R), on sensingnode 36 has exceeded the predetermined reference threshold voltage,v_(th). Other curves labeled “B” through “D” depict incrementalincreases of reference currents, i_(R), flowing through reference device32, resulting in correspondingly higher mirrored currents flowingthrough mirror device 34. This incrementally higher mirror currentrequires field strength current source 24 to source a higher currentlevel which in turn corresponds to higher values in counter 38. Thus, itis clear that our invention is adapted to effectively and efficientlydevelop a digital representation of the current flowing through sensingnode 36 that is suitable for any appropriate use.

One such use, as discussed earlier, of our field strength detector 20 isto cooperate with tuner 16 in controlling the operating characteristicsof the tank circuit 14. FIG. 6 illustrates one possible embodiment wherereceiver circuit 10 a uses a field strength detector 20 b speciallyadapted to share with tuner 16 a the control of the tank circuit 14. Inour Related References we have disclosed methods, and related apparatus,for dynamically tuning, via tuner 16 a, the tank circuit 14 so as todynamically shift the f_(R) of the tank circuit 14 to better match thef_(C) of the received RF signal at antenna 12. By way of example, wehave shown in FIG. 6 how the embodiment shown in FIG. 3 of our ParentPatent may be easily modified by adding to tuner 16 a a multiplexer 58to facilitate shared access to the tuner control apparatus. Shown inFIG. 7 is the operational flow (similar to that illustrated in FIG. 4 inour Parent Patent) of our new field strength detector 20 b upon assumingcontrol of tank circuit 14.

In context of this particular use, once tuner 16 a has completed itsinitial operating sequences as fully described in our Parent Patent, andour field strength detector 20 b has performed an initial sweep (asdescribed above and illustrated in FIG. 4) and saved in a differentiator60 a base-line field-strength value developed in counter 38,clock/control 44 commands multiplexer 58 to transfer control of the tankcircuit 16 a to field strength detector 20 b (all comprising step 62 inFIG. 7). Upon completing a second current sweep, differentiator 60 willsave the then-current field-strength value developed in the counter 38(step 64). Thereafter, differentiator 60 will determine the polarity ofthe change of the previously saved field-strength value with respect tothe then-current field-strength value developed in counter 38 (step 66).If the polarity is negative (step 68), indicating that the currentfield-strength value is lower than the previously-saved field-strengthvalue, differentiator 60 will assert a change direction signal;otherwise, differentiator 60 will negate the change direction signal(step 70). In response, the shared components in tuner 16 a downstreamof the multiplexer 58 will change the tuning characteristics of tankcircuit 14 (step 72) (as fully described in our Related References).Now, looping back (to step 64), the resulting change of field strength,as quantized is the digital field-strength value developed in counter 38during the next sweep (step 64), will be detected and, if higher, willresult in a further shift in the f_(R) of the tank circuit 14 in theselected direction or, if lower, will result in a change of direction(step 70). Accordingly, over a number of such ‘seek’ cycles, ourinvention will selectively allow the receiver 10 a to maximize receivedfield strength even if, as a result of unusual factors, the f_(R) of thetank circuit 14 may not be precisely matched to the f_(C) of thereceived RF signal, i.e., the reactance of the antenna is closelymatched with the reactance of the tank circuit, thus achieving maximumpower transfer. In an alternative embodiment, it would be unnecessaryfor tuner 16 a to perform an initial operating sequence as fullydescribed in our Parent Patent. Rather, field strength detector 20 b maybe used exclusively to perform both the initial tuning of the receivercircuit 10 a as well as the subsequent field strength detection. Notethat the source impedance of antenna 12 and load impedance of tankcircuit 14 may be represented alternatively in schematic form as in FIG.8, wherein antenna 12 is represented as equivalent source resistanceR_(S) 74 and equivalent source reactance X_(S) 76, and tank circuit 14is represented as equivalent load resistance R_(L) 78 and equivalent,variable load reactance XL 80.

In FIG. 9, we have illustrated alternate embodiments of our fieldstrength detector illustrated in FIG. 3. Here, as before, shunt circuit18 b is used to develop a substantially constant operating voltage levelacross supply node 28 and ground node 30. Also as before, the currentreference 22 is implemented as a current mirror circuit 22 b connectedin series with shunt circuit 18 b between nodes 28 and 30. However, inthis embodiment, the field strength current source comprises a resistivecomponent 84 adapted to function as a static resistive pull-up device.Many possible implementations exist besides a basic resistor, such as along channel length transistor, and those skilled in the art willappreciate the various implementations that are available to accomplishanalogous functionality. The field strength voltage reference v_(R)developed on sensing node 36 will be drawn to a state near the supplyvoltage when the mirrored current flowing though transistor 34 isrelatively small, e.g. close to zero amps, indicating a weak fieldstrength. As the field strength increases, the current flowing throughmirror transistor 34 will increase, and the field strength voltagereference v_(R) developed on sensing node 36 will drop proportionally tothe mirrored current flowing through mirror transistor 34 as i_(R)/N.ADC 40, having its input connected to sensing node 36, provides adigital output indicative of the field strength reference voltage,v_(R), developed on sensing node 36, as described previously.

In this alternate embodiment, latch 42 captures the output state of ADC40 in response to control signals provided by a clock/control circuit44. As disclosed earlier, the ADC 40 may comprise a comparator circuit.In this instance, ADC 40 is adapted to switch from a logic_1 state to alogic_0 when sufficient current is sunk by mirror transistor 34 to lowerthe voltage on sensing node 36 below a predetermined reference voltagethreshold, v_(th). Alternatively, ADC 40 may be implemented as amulti-bit ADC capable of providing higher precision regarding thespecific voltage developed on sensing node 36, depending on therequirements of the system.

Comparator 82 subsequently compares the captured output state held inlatch 42 with a value held in counter 38 that is selectively controlledby clock/control circuit 44. In response to the output generated bycomparator 82, clock/control circuit 44 may selectively change the valueheld in counter 38 to be one of a higher value or a lower value,depending on the algorithm employed. Depending upon the implementationof counter 38 and comparator 82, clock/control circuit 44 may alsoselectively reset the value of counter 38 or comparator 82 or both. Thedigital field-strength value developed by counter 38 is available forany appropriate use, as discussed above.

In FIG. 10, we have illustrated another alternate embodiment of ourfield strength detector illustrated in FIG. 3. Here, as before, shuntcircuit 18 c is used to develop a substantially constant operatingvoltage level across supply node 28 and ground node 30. In thisembodiment, the current reference 22 is implemented as a resistivecomponent 86 that functions as a static pull-down device. Many possibleimplementations exist besides a basic resistor, such as a long channellength transistor and those skilled in the art will appreciate thevarious implementations that are available to accomplish analogousfunctionality. The field strength voltage reference v_(R) developed onsensing node 36 will be drawn to a state near the ground node when thecurrent flowing though shunt circuit 18 c is relatively small, e.g.close to zero amps, indicating a weak field strength. As the fieldstrength increase, the current flowing through shunt circuit 18 c willincrease, and the field strength voltage reference v_(R) developed onsensing node 36 will rise proportionally to the current flowing throughshunt circuit 18 c. ADC 40, having its input connected to a sensing node36, provides a digital output indicative of the field strength referencevoltage, v_(R), developed on sensing node 36, as described previously.

In this alternate embodiment, latch 42 captures the output state of ADC40 in response to control signals provided by a clock/control circuit44. As disclosed earlier, the ADC 40 may comprise a comparator circuit.In this instance, ADC 40 is adapted to switch from a logic_0 state to alogic_1 when sufficient current is sourced by shunt circuit 18 c toraise the voltage on sensing node 36 above a predetermined referencevoltage threshold, v_(th). Alternatively, ADC 40 may be implemented as amulti-bit ADC capable of providing higher precision regarding thespecific voltage developed on sensing node 36, depending on therequirements of the system.

Comparator 82 subsequently compares the captured output state held inlatch 42 with a value held in counter 38 that is selectively controlledby clock/control circuit 44. In response to the output generated bycomparator 82, clock/control circuit 44 may selectively change the valueheld in counter 38 to be one of a higher value or a lower value,depending on the algorithm employed. Depending upon the implementationof counter 38 and comparator 82, clock/control circuit 44 may alsoselectively reset the value of counter 38 or comparator 82 or both. Thedigital field-strength value developed by counter 38 is available forany appropriate use, as discussed above.

In another embodiment, our invention may be adapted to sense theenvironment to which a tag is exposed, as well as sensing changes tothat same environment. As disclosed in our Related References, theauto-tuning capability of tuner 16 acting in conjunction with tankcircuit 14 detects antenna impedance changes. These impedance changesmay be a function of environmental factors such as proximity tointerfering substances, e.g., metals or liquids, as well as a functionof a reader or receiver antenna orientation. Likewise, as disclosedherein, our field strength (i.e., received power) detector 20 may beused to detect changes in received power (i.e., field strength) as afunction of, for example, power emitted by the reader, distance betweentag and reader, physical characteristics of materials or elements in theimmediate vicinity of the tag and reader, or the like. Sensing theenvironment or, at least, changes to the environment is accomplishedusing one or both of these capabilities.

As an example, the tag 88 of FIG. 11, contains both a source tag antenna12 (not shown, but see, e.g., FIG. 6) and a corresponding load chip tankcircuit 14 (not shown, but see, e.g., FIG. 6). Each contains bothresistive and reactive elements as discussed previously (see, e.g., FIG.8). A tag 88 containing such a tank circuit 14 mounted on a metallicsurface will exhibit antenna impedance that is dramatically differentthan the same tag 88 in free space or mounted on a container of liquid.Shown in Table 1 are exemplary values for impedance variations in bothantenna source resistance 74 as well as antenna source reactance 76 as afunction of frequency as well as environmental effects at an exemplaryfrequency.

TABLE 1 Antenna Impedance Variations 860 MHz 870 MHz 880 MHZ 890 MHz Rs,Ω Xs, Ω Rs, Ω Xs, Ω Rs, Ω Xs, Ω Rs, Ω Xs, Ω In Air 1.3 10.7 1.4 10.9 1.511.2 1.6 11.5 On Metal 1.4 10.0 1.5 10.3 1.6 10.6 1.7 10.9 On Water 4.911.3 1.8 11.1 2.4 11.7 2.9 11.5 On Glass 1.8 11.1 2.0 11.4 2.2 11.7 2.512.0 On Acrylic 1.4 10.6 1.6 11.1 1.7 11.4 1.9 11.7 900 MHZ 910 MHZ 920MHZ 930 MHZ Rs, Ω Xs, Ω Rs, Ω Xs, Ω Rs, Ω Xs, Ω Rs, Ω Xs, Ω In Air 1.811.8 2.0 12.1 2.2 12.4 2.4 12.8 On Metal 1.9 11.2 2.1 11.6 2.3 12.0 2.612.4 On Water 2.5 12.3 3.0 12.7 5.8 14.1 9.1 13.2 On Glass 2.8 12.4 3.212.8 3.7 13.2 4.2 13.6 On Acrylic 2.0 12.1 2.3 12.4 2.5 12.8 2.8 13.2

The tuner circuit 16 of our invention as disclosed in the RelatedReferences automatically adjusts the load impendence by adjusting loadreactance 80 (see, e.g., FIG. 8) to match source antenna impedancerepresented by source resistance 74 (see, e.g., FIG. 8) and sourcereactance 76 (see, e.g., FIG. 8). As previously disclosed, matching ofthe chip load impedance and antenna source impedance can be performedautomatically in order to achieve maximum power transfer between theantenna and the chip. My invention as disclosed in the RelatedReferences contained a digital shift register 90 for selectivelychanging the value of the load reactive component 80 (see, e.g., FIG.8), in the present case a variable capacitor, until power transfer ismaximized. (For reference, digital shift register 90 corresponds toshift register 64 in FIG. 5 of the Parent Patent.) This digital value ofthe matched impendence may be used either internally by the tag 88, orread and used by the reader 92, to discern relative environmentalinformation to which the tag 88 is exposed. For example, tag 88 maycontain a calibrated look-up-table within the clock/control circuit 44which may be accessed to determine the relevant environmentalinformation. Likewise, a RFID reader 92 may issue commands (seetransaction 1 in FIG. 11) to retrieve (see transaction 2 in FIG. 11) thevalues contained in digital shift register 90 via conventional means,and use that retrieved information to evaluate the environment to whichtag 88 is exposed. The evaluation could be as simple as referencingfixed data in memory that has already been stored and calibrated, or ascomplex as a software application running on the reader or its connectedsystems for performing interpretive evaluations.

Likewise, consider a tag 88 containing our field strength (i.e.,received power) detector 20 (not shown, but, e.g., see FIG. 6) whereinthe method of operation of the system containing the tag 88 calls forour field strength detector 20 to selectively perform its sweep functionand developing the quantized digital representation of the current viathe method discussed earlier. As illustrated in FIG. 11, counter 38 willcontain the digital representation developed by our field strengthdetector 20 of the RF signal induced current, and may be used eitherinternally by the tag 88, or read and used by the reader 92, to discernrelative environmental information to which the tag 88 is exposed. Forexample, reader 92 may issue a command to the tag 88 (see transaction 1in FIG. 11) to activate tuner 16 and/or detector 20 and, subsequent tothe respective operations of tuner 16 and/or detector 20, receive (seetransaction 2 in FIG. 11) the digital representations of either thematched impedance or the maximum current developed during thoseoperations. Once again, this digital value of the field strength storedin the counter 38 may be used either internally by the tag 88, or readand used by the reader 92, to discern relative environmental informationto which the tag 88 is exposed. For example, tag 88 may contain acalibrated look-up-table within the clock and control block 44 which maybe accessed to determine the relevant environmental information.Likewise, a RFID reader may issue commands to retrieve the valuescontained in digital shift register 90, and use that retrievedinformation to evaluate the environment to which tag 88 is exposed. Theevaluation could be as simple as referencing fixed data in memory thathas already been stored and calibrated, or as complex as a softwareapplication running on the reader or its connected systems forperforming interpretive evaluations. Thus, the combining of thetechnologies enables a user to sense the environment to which a tag 88is exposed as well as sense changes to that same environment.

As we have explained in the Parent Provisional One, it is well knownthat changes in some environmental factors will result in respectivechanges the effective impedance of the antenna 12. In a number of theRelated References, we have shown that it is possible to dynamicallyretune the tank circuit 14 to compensate for the environmentally-inducedchange in impedance by systematically changing the digital tuningparameters of tank circuit 14, using techniques disclosed, inter alia,in Parent Patent One. We will now show how it is possible to develop anestimate of the relative change in the environmental factor as afunction of the relative change in the digital tuning parameters of thetank circuit 14.

As can be seen in Table 1, above, it is possible to develop, a priori, areference table storing information relating to a plurality ofenvironmental reference conditions. Thereafter, in carefully controlledconditions wherein one and only one environmental condition of interestis varied (see, FIG. 12), an operational tag 88 is exposed to each ofthe stored reference conditions (step 94) and allowed to complete thetank tuning process. (recursive steps 96 and 98). After tuning hasstabilized, the tag 88 can be interrogated (step 100), and the finalvalue in the shift register 90 retrieved (step 100). This value is thenstored in the reference table in association with the respectiveenvironmental condition (step 102). The resulting table might look likethis:

TABLE 2 Tuning Parameters vs. Frequency 860 870 880 890 900 910 920 930MHz MHz MHz MHz MHz MHz MHz MHz In Air 25 21 16 12  8 4 0 0* On Metal 3127 22 17 12 8 3 0 On Water 20 19 12 12  4 0 0* 0* On Glass 21 17 12  8 4 0* 0* 0* On Acrylic 23 19 14 10  6 2 0* 0* 0* indicates that a lowercode was needed but not available; 0 is a valid code.

In contrast to prior art systems in which the antenna impedance must beestimated indirectly, e.g., using the relative strength of the analogsignal returned by a prior art tag 88 in response to interrogation bythe reader 92, our method employs the on-chip re-tuning capability ofour tag 88 to return a digital value which more directly indicates theeffective antenna impedance. Using a reference table having asufficiently fine resolution, it is possible to detect even modestchanges in the relevant environmental conditions. It will be readilyrealized by practitioners in this art that, in general applications,environment conditions typically do not change in an ideal manner, and,more typically, changes in one condition are typically accompanied bychanges in at least one other condition. Thus, antenna design will beimportant depending on the application of interest.

As noted in our Parent Provisional Two, one possible approach would beto mount the antenna 12 on a substrate that tends to amplify theenvironmental condition of interest, e.g., temperature.

Shown in FIG. 13 is an RF sensing system 104 constructed in accordancewith one embodiment of our invention, and specially adapted tofacilitate sensing of one or more environmental conditions in a selectedregion surrounding the system 104. In general, the system 104 comprises:an RF transceiver 106; a di-pole antenna 108 comprising a pole 108 a andan anti-pole 108 b; and a tail 110 of effective length T, comprisingrespective transmission line pole 110 a and transmission line anti-pole110 b, each of length T/2. In accordance with our invention, thedifferential transmission line elements 110 a-110 b are symmetricallycoupled to respective poles 108 a-108 b at a distance d from the axis ofsymmetry of the antenna 108 (illustrated as a dotted line extendinggenerally vertically from the transceiver 106). In general, d determinesthe strength of the interaction between the transmission line 110 andthe antenna 108, e.g., increasing d tends to strengthen the interaction.In the equivalent circuit shown in FIG. 13b , the voltage differentialbetween the complementary voltage sources 108 a and 108 b tends toincrease as d is increased, and to decrease as d is decreased.Preferably d is optimized for a given application. However, it will berecognized that the sensitivity of the antenna may be degraded as afunction of d if a load, either resistive or capacitive, is imposed onthe tail 110.

In operation, the tail 110 uses the transmission line poles 110 a-110 bto move the impedance at the tip of the tail 110 to the antenna 108,thus directly affecting the impedance of the antenna 108. Preferably,the transceiver 106 incorporates our tuning circuit 16 so as to detectany resulting change in antenna impedance and to quantize that changefor recovery, e.g., using the method we have described above withreference to FIG. 12.

By way of example, we have illustrated in FIG. 14 one possibleembodiment of the system 104 in which the antenna poles 108 a-108 b areinstantiated as a patch antenna (illustrated in light grey), with theantenna pole 108 a connected to one output of transceiver 106, and theother output of transceiver 106 connected to the antenna anti-pole 108b. A ground plane 112 a (illustrated in a darker shade of grey than thepatch antenna 108) is disposed substantially parallel to both theantenna poles 108 a-108 b and a ground plane 112 b disposedsubstantially parallel to the transmission line poles 110 a-110 b. As isknown, the ground planes 112 are separated from the poles by adielectric substrate (not shown), e.g., conventional flex material orthe like. If the dielectric layer between the antenna poles 108 andground plane 112 a is of a different thickness than the layer betweenthe transmission line poles 110 and the ground plane 112 b, the groundplane 112 b may be disconnected from the ground plane 112 a and allowedto float. In general, this embodiment operates on the same principles asdescribed above with reference to FIG. 13.

Shown in FIG. 15 is an antenna 114 constructed in accordance with oneother embodiment of our invention, and specially adapted for use in thesensing system 104 to facilitate sensing the presence of fluids; and, inparticular, to the depth of such fluids. In the illustrated embodiment,antenna 114 comprises a head portion 116 and a tail portion 118. Ingeneral, the head 116 is adapted to receive RF signals and to transmitresponses using conventional backscatter techniques; whereas the tailportion 118 functions as a transmission line. During normal operation,the tail 118 acts to move and transform the impedance at the tip of thetail to the head 116. Accordingly, any change in the tip impedance dueto the presence of fluid will automatically induce a concomitant changein the impedance of the head antenna 116. As has been explained above,our tuning circuit 16 will detect that change and re-adjust itself so asto maintain a reactive impedance match. As has been noted above, anysuch adjustment is reflected in changes in the digital value stored inshift register 90 (FIG. 11).

Shown in FIG. 16 is one possible flow for a sensing system 104 using theantenna 114. As has been explained above with reference to FIG. 12, thesensor is first calibrated (step 120) to detect the presence of varyinglevels of a particular substance. For the purposes of this discussion,we mean the term substance to mean any physical material, whetherliquid, particulate or solid, that is: detectable by the sensor; and towhich the sensor demonstrably responds. By detectable, we mean that,with respect to the resonant frequency of the antenna 114 in the absenceof the substance, the presence of the substance in at least somenon-trivial amount results in a shift in the resonant frequency of theantenna 114, thereby resulting in a concomitant adjustment in the valuestored in the shift register 90; and by demonstrably responds we meanthat the value stored in the shift register 90 varies as a function ofthe level the substance relative to the tip of the tail 118 of theantenna 114. Once calibrated, the sensor can be installed in a structure(step 122), wherein the structure can be open, closed or any conditionin between. The structure can then be exposed to the substance (step124), wherein the means of exposure can be any form appropriate for boththe structure and the substance, e.g., sprayed in aerosol, foam or dustform, immersed in whole or in part in a liquid, or other known forms.Following a period of time deemed appropriate for the form of exposure,the sensor is interrogated (step 126) and the then-current value storedin the shift register 90 retrieved. By correlating this value with thetable of calibration data gathered in step 120, the presence or absenceof the substance can be detected (step 128).

In one embodiment, the table of calibration data can be stored in thesensor and selectively provided to the reader during interrogation toretrieve the current value. Alternatively, the table can be stored in,e.g., the reader and selectively accessed once the current value hasbeen retrieved. As will be clear, other embodiments are possible,including storing the table in a separate computing facility adapted toselectively perform the detection lookup when a new current value hasbeen retrieved.

Assume by way of example, an automobile assembly line that includes asan essential step the exposure, at least in part, of apartially-assembled automobile chassis to strong streams of a fluid,e.g., water, so as to determine the fluid-tightness of the chassis.Given the complexity of a modern automobile, it is not cost effective tomanually ascertain the intrusion of the fluid at even a relatively smallnumber of possible points of leakage. However, using our sensors andsensing system 104, we submit that it is now possible to installrelatively large numbers of independently operable sensors during theassembly process, even in highly inaccessible locations such aslargely-enclosed wiring channels and the like. In the course of suchinstallations, the unique identity codes assigned to each installedsensor is recorded together with pertinent installation locationdetails. After extraction from the immersion tank, the chassis can bemoved along a conventional conveyor path past an RFID reader sited in aposition selected to facilitate effective querying of all of theinstalled sensors. In one embodiment, the reader may be placed above themoving chassis so as to “look down” through the opening provided for thefront windshield (which may or may not be installed) into the interiorportion of the chassis; from such a position even those sensorsinstalled in the “nooks and crannies” in the trunk cavity should bereadable. By correlating the code read from each sensor with thepreviously constructed, corresponding table, it is now possible todetect the presence (or absence) of the substance at the respectivelocation of that sensor; indeed, if the sensor is sufficiently sensitiveto the substance, it may be possible to estimate the severity of theleakage in the vicinity of each sensor.

Shown in FIG. 17 is an antenna 130 constructed in accordance with oneother embodiment of our invention, and specially adapted for use in thesensing system 104 to facilitate sensing the presence of fluids; and, inparticular, to the depth of such fluids. As illustrated in FIG. 17a ,the top layer of antenna 132 comprises: a patch antenna portion 134; anantenna ground plane 136; a tail portion 138; and a die attach area 140.As noted in FIG. 17a , the tail portion 138 of antenna 130 comprises apair of generally parallel transmission lines 142, each substantiallythe same in length. As illustrated in FIG. 17b , the bottom layer ofantenna 130 comprises a ground plane 136 for the transmission lines 142.During a typical assembly process, the illustrated shapes are formed inthe top and bottom layers of a continuous roll of copper-clad flexcircuit material, and each antenna 130 cut from the roll using a rollingcutter assembly. An RFID tag device (incorporating our tuning circuit16) is then attached to the die attach area 140, and the antenna 130 isfolded along fold lines 1 and 2 generally around a suitable corematerial such as PET or either open-cell or closed-cell foam.

In general, the patch antenna portion 134 is adapted to receive RFsignals and to transmit responses using conventional backscattertechniques. During normal operation, the transmission lines 142comprising the tail 138 act to move and transform the impedance at thetip of the tail 138 to the patch antenna 134. Accordingly, any change inthe tip impedance due to the presence of fluid will automatically inducea concomitant change in the impedance of the head antennal. As has beenexplained above, our tuning circuit 16 will detect that change andre-adjust itself so as to maintain a reactive impedance match. As hasbeen noted above, any such adjustment is reflected in changes in thedigital value stored in shift register 90 (FIG. 11).

Thus it is apparent that we have provided an effective and efficientmethod and apparatus for sensing changes to an environment to which theRFID tag is exposed. Those skilled in the art will recognize thatmodifications and variations can be made without departing from thespirit of our invention. Therefore, we intend that our inventionencompass all such variations and modifications as fall within the scopeof the appended claims.

What is claimed is:
 1. A radio frequency identification (RFID) tagcomprises: an antenna operable to receive a radio frequency (RF) signal,wherein the RF signal has a carrier frequency of a plurality of carrierfrequencies, wherein the plurality of frequencies spans a broadfrequency band; a tank circuit coupled to the antenna, wherein an inputimpedance of the RFID tag is based on one or more of impedance of theantenna and impedance of the tank circuit; and a tuning circuit operablycoupled to adjust, in increments, the input impedance until asubstantially maximum power level of the received RF signal is achieved,wherein the substantially maximum power level corresponds to the carrierfrequency being substantially equal to a resonant frequency of at leastone of the antenna and the tank circuit.
 2. The RFID tag of claim 1,wherein the tuning circuit is further operable to adjust the inputimpedance by: providing a control signal to the tank circuit, wherein,based on the control signal, the tank circuit produces an adjusted tankcircuit impedance; determining a most recent power level of the receivedRF signal with the tank circuit having the adjusted tank circuitimpedance; comparing the most recent power level with one or moreprevious power levels of the received RF signal, wherein a previouspower level of the one or more previous power levels is determined withthe tank circuit having a previously adjusted tank circuit impedance,wherein the tuning circuit provided a previous control signal to thetank circuit causing the tank circuit to produce the previously adjustedtank circuit impedance; and selecting one of a plurality of controlsignals that corresponds to the received RF signal having a mostfavorable power level to adjust the input impedance to produce thesubstantially maximum power level of the received RF signal, wherein theplurality of control signals includes the control signal and theprevious control signal.
 3. The RFID tag of claim 1, wherein the tuningcircuit is further operable to adjust the input impedance by: decreasingthe impedance of the tank circuit to produce an adjusted inputimpedance; determining a most recent power level of the received RFsignal based on the adjusted input impedance; comparing the most recentpower level with a previous power level of the received RF signal; andwhen the most recent power level is lower than the previous power level:incrementally adjusting the impedance of the tank circuit whilecorresponding incremental power levels of the received RF signal areincreasing; and when a current incremental power level of thecorresponding incremental power levels is less than an immediatelypreceding incremental power level of the corresponding incremental powerlevels, utilizing the adjusted impedance of the tank circuitcorresponding to the immediately preceding incremental power level asthe adjustment of the input impedance to produce the substantiallymaximum power level of the received RF signal.
 4. The RFID tag of claim3 further comprises: when the most recent power level is greater thanthe previous power level: incrementally adjusting the impedance of thetank circuit while corresponding incremental power levels of thereceived RF signal are increasing; and when a current incremental powerlevel of the corresponding incremental power levels is less than animmediately preceding incremental power level of the correspondingincremental power levels, utilizing the adjusted impedance of the tankcircuit corresponding to the immediately preceding incremental powerlevel as the adjustment of the input impedance to produce thesubstantially maximum power level of the received RF signal.
 5. The RFIDtag of claim 1, wherein the tuning circuit is further operable to adjustthe input impedance by: increasing the impedance of the tank circuit toproduce an adjusted input impedance; determining a most recent powerlevel of the received RF signal based on the adjusted input impedance;comparing the most recent power level with a previous power level of thereceived RF signal; and when the most recent power level is lower thanthe previous power level: incrementally adjusting the impedance of thetank circuit while corresponding incremental power levels of thereceived RF signal are increasing; and when a current incremental powerlevel of the corresponding incremental power levels is less than animmediately preceding incremental power level of the correspondingincremental power levels, utilizing the adjusted impedance of the tankcircuit corresponding to the immediately preceding incremental powerlevel as the adjustment of the input impedance to produce thesubstantially maximum power level of the received RF signal.
 6. The RFIDtag of claim 5 further comprises: when the most recent power level isgreater than the previous power level: incrementally adjusting theimpedance of the tank circuit while corresponding incremental powerlevels of the received RF signal are increasing; and when a currentincremental power level of the corresponding incremental power levels isless than an immediately preceding incremental power level of thecorresponding incremental power levels, utilizing the adjusted impedanceof the tank circuit corresponding to the immediately precedingincremental power level as the adjustment of the input impedance toproduce the substantially maximum power level of the received RF signal.7. The RFID tag of claim 1, wherein the tank circuit comprises: aninductor; and a variable capacitor, wherein the tuning circuit isoperable to adjust the variable capacitor.
 8. The RFID tag of claim 1,wherein the tank circuit comprises: a capacitor; and a variableinductor, wherein the tuning circuit is operable to adjust the variableinductor.
 9. The RFID tag of claim 1 further comprises: a regulatoroperable to convert the received RF signal into a power supply voltagethat powers the tuning circuit.
 10. The RFID tag of claim 1, wherein thetuning circuit comprises: a reference circuit operable to generate apower reference of the received RF signal; a comparison circuit operableto compare a current power reference of a current power level of thereceived RF signal with a previous power reference of a previous powerlevel of the received RF signal; and control signal circuit operablecoupled to generate a control signal based on the comparing of thecurrent power reference with a previous power reference.
 11. The RFIDtag of claim 10 further comprises: the reference circuit including avoltage regulator to produce a current voltage as the current powerreference and a previous voltage as the previous power reference; thecomparison circuit including: a differentiator operable to determine apolarity change between the current voltage and the previous voltage; aselector operable to generate an up signal or a down signal based on thepolarity change; and the control signal circuit including a ramp circuitoperable to generate the control signal based on the up signal or thedown signal.
 12. The RFID tag of claim 10 further comprises: thereference circuit including: a rectifier operable to rectify thereceived RF signal to produce a rectified signal; and a low pass filteroperable to filter the rectified signal to produce the power reference;the comparison circuit including: a sample & hold circuit operable tosample and hold the power reference, wherein an input to the sample &hold circuit corresponds to the current power reference and an output ofthe sample & hold circuit corresponds to the previous power reference; acomparator operable to compare the current power reference with theprevious power reference to produce a comparative output; and a latchoperable to temporarily store the comparative output; and the controlsignal circuit including a shift register operable to generate thecontrol signal from the latched comparative output.
 13. A method forexecution by a radio frequency identification (RFID) tag, the methodcomprises: receiving, from an RFID reader, a radio frequency (RF)signal, wherein the RF signal has a carrier frequency of a plurality ofcarrier frequencies, wherein the plurality of frequencies spans a broadfrequency band; adjusting input impedance of the RFID tag over a rangeof input impedances, wherein the input impedance of the RFID tag isbased on one or more of impedance of an antenna of the RFID tag andimpedance of a tank circuit of the RFID tag; monitoring power level ofthe received RF signal over the range of input impedances; and selectingthe input impedance of the range of input impedances providing asubstantially maximum power level of the received RF signal, wherein thesubstantially maximum power level corresponds to the carrier frequencybeing substantially equal to a resonant frequency of at least one of theantenna and the tank circuit.
 14. The method of claim 13, wherein theselecting the input impedance of the range of input impedances furthercomprises: providing a control signal to the tank circuit, wherein,based on the control signal, the tank circuit produces an adjusted tankcircuit impedance; determining a most recent power level of the receivedRF signal with the tank circuit having the adjusted tank circuitimpedance; comparing the most recent power level with one or moreprevious power levels of the received RF signal, wherein a previouspower level of the one or more previous power levels is determined withthe tank circuit having a previously adjusted tank circuit impedance,wherein a previous control signal was provided to the tank circuitcausing the tank circuit to produce the previously adjusted tank circuitimpedance; and selecting one of a plurality of control signals thatcorresponds to the received RF signal having a most favorable powerlevel to adjust the input impedance to produce the substantially maximumpower level of the received RF signal, wherein the plurality of controlsignals includes the control signal and the previous control signal. 15.The method of claim 13, wherein the selecting the input impedance of therange of input impedances further comprises: decreasing the impedance ofthe tank circuit to produce an adjusted input impedance; determining amost recent power level of the received RF signal based on the adjustedinput impedance; comparing the most recent power level with a previouspower level of the received RF signal; and when the most recent powerlevel is lower than the previous power level: incrementally adjustingthe impedance of the tank circuit while corresponding incremental powerlevels of the received RF signal are increasing; and when a currentincremental power level of the corresponding incremental power levels isless than an immediately preceding incremental power level of thecorresponding incremental power levels, utilizing the adjusted impedanceof the tank circuit corresponding to the immediately precedingincremental power level as the adjustment of the input impedance toproduce the substantially maximum power level of the received RF signal.16. The method of claim 15 further comprises: when the most recent powerlevel is greater than the previous power level: incrementally adjustingthe impedance of the tank circuit while corresponding incremental powerlevels of the received RF signal are increasing; and when a currentincremental power level of the corresponding incremental power levels isless than an immediately preceding incremental power level of thecorresponding incremental power levels, utilizing the adjusted impedanceof the tank circuit corresponding to the immediately precedingincremental power level as the adjustment of the input impedance toproduce the substantially maximum power level of the received RF signal.17. The method of claim 13, wherein the selecting the input impedance ofthe range of input impedances further comprises: increasing theimpedance of the tank circuit to produce an adjusted input impedance;determining a most recent power level of the received RF signal based onthe adjusted input impedance; comparing the most recent power level witha previous power level of the received RF signal; and when the mostrecent power level is lower than the previous power level: incrementallyadjusting the impedance of the tank circuit while correspondingincremental power levels of the received RF signal are increasing; andwhen a current incremental power level of the corresponding incrementalpower levels is less than an immediately preceding incremental powerlevel of the corresponding incremental power levels, utilizing theadjusted impedance of the tank circuit corresponding to the immediatelypreceding incremental power level as the adjustment of the inputimpedance to produce the substantially maximum power level of thereceived RF signal.
 18. The method of claim 17 further comprises: whenthe most recent power level is greater than the previous power level:incrementally adjusting the impedance of the tank circuit whilecorresponding incremental power levels of the received RF signal areincreasing; and when a current incremental power level of thecorresponding incremental power levels is less than an immediatelypreceding incremental power level of the corresponding incremental powerlevels, utilizing the adjusted impedance of the tank circuitcorresponding to the immediately preceding incremental power level asthe adjustment of the input impedance to produce the substantiallymaximum power level of the received RF signal.